The invention relates to a system for automatically compensating for positioning errors of a robot in all degrees of freedom.
In robot applications the robot gripper is under computer control programmed to successively and repeatedly be positioned and operated in predetermined positions within the work envelope of the robot. Reference is made to an IBM Robot System marketed by IBM as the IBM 7565 Manufacturing System which is well known to those familar with the robotic art.
At the start up of a new work process it is necessary to let the gripper start from an exactly defined home position. Then during operation of the robot, errors in the positioning of the gripper may occur due to errors in some or all of the six coordinate directions. Therefore, calibration and compensation of the robot in all coordinate directions is necessary at suitable time intervals.
Such calibrations have previously been performed by means of separate measurements for each coordinate direction and at different points depending on the coordinate direction to be measured. Then, from the values received, corrections are made in the separate coordinate directions. Such measurements are very time consuming. Besides, as the direction coordinates depend on each other repeated measurements must be performed to get satisfactory precision.
U.S. Pat. No. 4,362,977, issued Dec. 7, 1982, describes such a method and apparatus for calibration of a robot and uses the results of the calibration to compensate for inaccuracies of the robot. The method includes moving the robot gripper to a set of nominal positions, commanded by the robot controller or measured by position encoders built into the robot, and determining the associated actual positions by measuring the robot position with an independent accurate measuring means or by aligning the robot end effector with an accurate calibration mask. The calibration results are stored and subsequently used to compensate for robot inaccuracy so that the robot moves to the actual desired positions.